1. Field of the Invention
The invention is related generally to the field of electrical resistivity well logging methods. More specifically, the invention is related to a method and apparatus for providing a transverse coil and measuring cross-component magnetic fields in a downhole resisitivity tool.
2. Description of the Related Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, reasonably determine the petrophysical properties of the formation and the fluids therein.
The physical principles of electromagnetic induction resistivity well logging are described, for example, in H.G. Doll, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil-Based Mud, Journal of Petroleum Technology, vol. 1, p. 148, Society of Petroleum Engineers, Richardson, Tex. (1949). Many improvements and modifications to electromagnetic induction resistivity instruments have been devised since publication of the Doll reference, supra. Examples of such modifications and improvements can be found, for example, in U.S. Pat. No. 4,837,517; U.S. Pat. No. 5,157,605 issued to Chandler et al.; and U.S. Pat. No. 5,452,761 issued to Beard et al.
A typical electrical resistivity-measuring instrument is an electromagnetic induction military well logging instrument such as described in U.S. Pat. No. 5,452,761 issued to Beard et al. The induction logging instrument described in the Beard '761 patent includes a number of receiver coils spaced at various axial distances from a transmitter coil. Alternating current is passed through the transmitter coil, which induces alternating electromagnetic fields in the earth formations. Voltages, or measurements, are induced in the receiver coils as a result of electromagnetic induction phenomena related to the alternating electromagnetic fields. A continuous record of the voltages form curves, which are also referred to as induction logs. The induction instruments that are composed of multiple sets of receiver coils are referred to as multi-array induction instruments. Every set of receiver coils together with the transmitter is named as a subarray. Hence, a multi-array induction consists of numerous subarrays and acquires measurements with all the subarrays.
Conventional induction tools comprising only coaxial transmitter-receiver coil configurations do not have azimuthal sensitivity. Therefore, in a horizontal wellbore, the data do not contain information about directionality of the formation. therefore, It is not possible to distinguish whether a layer is above or below the borehole from these data alone. There is a need to be able to determine directionality of the formation for use in, e.g., geosteering. This directionality knowledge can be obtained using a subset or all of the cross-components of the new multi-component induction tool to allow determination of directionality of the formation.